Measuring and control apparatus



Jan. 2, 1945. T. R. HARRISON MEASURING AND CONTROL APPARATUS Filed May 14, 1942 5 Sheets-Sheet 1 -o c 27 2g RECON- VERTER o c INVENT OR.

THOMAS R. HARRISON Jan. 2, 1945. T. R. HARRISON 2,366,618

MEASURING AND CONTROL APPARATUS Filed May 14, 1942 5 Sheets-Sheet 2 5{ H9 23 zmnm;

INVENTOR. a

THOMAS R.HARRISON ATTORNEY.

Jan. 2, 1945. "r. R. HARRISON MEASURING AND CONTROL APPARATUS Filed May 14, 1942 5 Sheets-Sheet 5 INVENTOR. 23 THOMAS RHARRISON.

A ORNEY.

Jane 2, 1945. T. R. HARRISON 2,366,618

MEASURING AND CONTROL APPARATUS Filed May 14, 1942 5 Sheets-Sheet 4 INVENTOR.

A THOMAS R HARRISON L1 BY A TORNEY.

Jan. 2, 1945. 'r. R. HARRISON MEASURING AND CONTROL APPARATUS Filed May 14, 1942 5 Sheets-Sheet 5 FIG.|2.

INVENTOR.

THOMAS R. HARRSON TTORNEY.

Patented Jan. 2, 1945 MEASURING AND CONTROL APPARATUS Thomas B. Harrison, Wyncote, Pa., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application May 14, 1942, Serial No. 442,999

21 Claims.

The present invention relates to apparatus.

having articular utility in measuring and/or controlling the magnitud and changes in magnitude of a variable condition, and more specifically, to self balancing electrical apparatus which is useful in measuring and/or controlling the magnitude of electrical, thermal, chemical, physical or mechanical quantities or qualities such as electric current, temperature pressure, flow or hydrogen ion content. The present invention is especially useful in pyrometric measuring and controlling apparatus for measuring voltage changes of thermocouples and photovoltaic cells and the like which are exposed to variations in temperature or radiant energy and for eflfecting desired control operations in accordance with such measurements. The invention is also useful i many other diflerent and'varied applications among which may be included measuring and controlling electrolytic conditions of industrial cells and particularly the determination of ydrogen ion content values of cells wherein it is desired to measure small voltag variations accurately and automatically.

An object of the invention is to provide improved measuring apparatus which is capable of continuously indicating, recording and/or controlling the magnitude and changes in magnitude of a variable condition substantially instantameasuring ranges so as to enable a higher degree of accuracy of measurement to be attained.

Another specific object of the invention is to provide improved apparatus for measuring and/or utilizing for control and analogous purposes minute direct currents such as those derived from thermocouples or photovoltaic cells.

In accordance with one embodiment of the present invention a source of direct electric potential to be measured is conductively connected in circuit with a fixed resistance which is traversed by direct current of predetermined magnitud derived -1'rom a suitable current source. The potential drop produced across the fixed resistance by the derived direct current is opposed to the direct electric potential to be measured and the derived direct electric potential is preferably made adjustable so that the two potentials may be made to cancel each other out when the electric potential to be measured is approximately midway its normal range of variation. The diflerence in potential between the two opp ed potentials is transformed by a suitable converter into an alternating current which may be readily amplified and the alternating current so derived is impressed on the input circuit of an electronic amplifier, th output circuit 01 which is also electrically connected to the fixed res stance. The transformation of the difneously with the occurrence of such variations. ferential potential into an alternating voltage is Another and more specific object of the invention is to provide self balancing apparatus which is characterized in that the condition responsive means does not require the use of physically movable arts deflecting in accordance with the variations in the condition under measurement whereby the apparatus is capable of measuring accurately the magnitude and changes in magnitude of rapidly varying conditions. An-

other advantage which is obtained as a result of dispensing with the use of such physically movable parts in the apparatus, which parts in the prior art apparatus are usually extremely delicate and expensive, is that the apparatus is rendered substantially less sensitive to the of 5 affected by periodically interrupting the difierential potential at regular frequency and applying the interrupted quantity to the primary winding of a transformer. By virtue of the action of the transformer an alternating voltage of one phase or of opposite phase depending upon the polarity of the potential difference is induced in the transformer secondary winding. The magnitude of the alternating voltage thus created in the transformer secondary winding varies in accordance with the magnitude of the diflerential potential.

The derived alternating voltag is impressed on the input circuit of the electronic amplifier and the amplified alternating current is reconverted into a direct current of one polarity or of opposite polarity depending upon the phase of the derived alternating voltage. The direct current output of the reconverter changes the flow of current through the fixed resistance as required to reduce the diflference in potential between the otential under measurement and the predetermined potential substantially to zero. The current change in the fixed resistance required to cancel out the diflerential potential will vary in accordance with the variations in the potential under measurement and may be measured by suitable indicating and/or recording devices to provide an indication and/or record of the variations in the potential of the source under measurement.

Those skilled in the art will recognize from the foregoing explanation and the detailed description given hereinafter that I have provided a self balancing potentiometric instrument which is characterized by the fact that the condition responsive means does not require, nor include, any movable parts deflecting in accordance with th variations in the potential under measurement, and in addition, rebalance of the potentiometer is also effected without requiring the use of any physically movable parts. Furthermore, the rebalancing operation is performed in such manner as to provide a relatively large electric current which varies in direct proportionality with the potential under measurement and may be utilized for the operation of relatively rugged indicating and/or recording apparatus. Such apparatus is especially adapted for accurately indicating and/or recording the variations in rapidly varying conditions. In addition, suitable provisions have been made for varying the range of measurement of the apparatus so as to enable a higher degree of accuracy to be attained.

The various features of novelty which characterize my present invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and the specific objects obtained with its use, reference should be had to the accompanying drawings and descriptive matter in which are illustrated and described preferred embodiments of the invention.

Of the drawings:

Fig. 1 is a diagrammatic representation of one embodiment of the invention;

Figs. 2, 3 and 4 illustrate in detail one form of converter which may be utilized in the arrangement of Fig. 1;

Fig. 5 illustrates in detail one form of electronic amplifier which may be utilized for amplifying the alternating current derived by means of the converter shown in Figs. 2-4;

Fig. 6 is a diagrammatic illustration of one firm of reconverter which may be utilized for translating the alternating current output of the amplifier into a direct current of one polarity or of the opposite polarity depending upon the phase of the amplifier alternating current outp Figs. 7, 8, 9, 10 and 11 each illustrate a different modification of the arrangement of Fig. 1; and

Figs. 12 and 13 are diagrammatic representations of the use of the arrangement of Fig. 1 in a control system.

Referring now to Fig. 1 of the drawings, there is illustrated in schematic form an arrangement for indicating and recording the temperature of a furnace I in the interior of which a thermocouple 2 is arranged so as to be responsive to slight changes in temperature therein. The thermocouple, which may be located at a distance from the remainder of the measuring apparatus, is connected by a pair of conductors 3 and 4 into a potentiometric network indicated generally by the reference numeral 5. The potentiometric network 5 also has connected therein a fixed resistance 5, an interrupter or converting device I and the primary winding I of a transformer 9 having secondary winding ll. The interrupter or converting device 1 is described in detail hereinafter in connection with Figs. 2, 3 and 4. In the arrangement of Fig. 1 the electromotive force developed by the thermocouple 2 is opposed to the potential drop produced across the fixed resistance 8 by a flow of direct current of predetermined magnitude therethrough. The direct current of predetermined magnitude through the resistance 0 is derived from alternating current mains II and H by means of a rectifier I3 and an adjustable resistance H which are connected in series across the alternating current mains II and i2. A condenser I5 is connected in shunt to resistance II. The rectifier II, which is a half wave rectifier and may be of the copper oxide type, is connected in series with the resistance ll across the alternating current supply mains II and I2. Since the rectifier I3 is adapted to conduct current in one direction only, a unidirectional potential drop is produced across the resistance I4 and a portion of this potential drop depending upon the position of contact ll along the resistance I4 is utilized for creating a current flow through resistance 6 and thereby a potential drop across the latter which is opposed to the electromotive force developed by the thermocouple 2 through a circuit which may be traced from one terminal of thermocouple 2, to conductor 2, interrupter I and transformer primary winding I, conductor ll, resistance 8, and conductor 4 to the other terminal of thermocouple 2. The portion of the potential drop tapped oil the resistance It may be adjusted as desired by means of the manipulation of a knob i1 and as shown a scale ll disposed alongside the resistance may be provided for the purpm of indicating the portion of the potential drop tapped oi! the resistance H. The condenser II is provided for smoothing out the pulsations or ripple in the unidirectional potential drop maintained across the resistance N so that a substantially smooth direct current flow is created in the resistance 8 for producing a steady potential drop in opposition to the electromotive force produced by the thermocouple 2. The condenser I5 is employed as a filter. It will be apparent that if additional stages of filtering are required such additional stages of filtering can be utilized.

The current established in resistance 6 by the variable part of the potential drop tapped oil the resistance ll flows through a circuit which may be traced from the contact I6 through a resistance 25, resistance 6, a resistance 20, a conductor 2| in which a milliammeter recorder 22 and a milliammeter indicator 23 are connected, and a conductor 2| to the lower terminal of the resistance H. The contact I6 is preferably so adjusted along the length of the resistor N that the potential drop created across resistance 6 by the flow of current therethrough is approximately equal to that electromotive force which is midway the normal range of variation of the thermocouple electromotive force. At this value of the thermocouple electromotive force the opposed potentials are exactly equal and opposite, and therefore, the difierence between those potentials is then zero. The current flow through resistance 6 required to establish this condition will be indicated by the milliammeter recorder 22 and the milliammeter indicator 23.

In accordance with the arrangement of Fig. l the difference in potential between that developed by the thermocouple 2 and that produced across resistance 6 is translated by means of the interrupter 1 and the transformer 8 into an alternating current of one phase or of opposite phase depending upon the polarity of the said differential potential and thereby upon the direction of the change in the electromotive force produced by the thermocouple 2. The alternating potential so derived in the secondary winding III of the transformer 9 also varies in accordance with the magnitude of the differential potential and is applied to the input terminals of an electronic amplifier 26 which is described in detail in connection with Fig. 5. The alternating current output of the electronic amplifier 26 is applied to the primary winding 21 of a transformer 28 having a secondary winding 29. The secondary winding 29 is connected to the input terminals of an electronic device or reconverter 30, described in detail in connection with Fig. 6, which operates to translate the alternating current impressed on its input terminals into a direct current of one polarity or of the opposite polarity depending upon the phase of the impressed alternating current. In addition the amplitude of the direct current output of the electronic device 30 varies in accordance with the magnitudeof the alternating current impressed on the input terminals thereof. A condenser'3i is connected across the output terminals of the device 30 for the purpose of smoothing out the pulsations or ripple in the direct current output of the device 30, and if desired, additional stages of filtering may be provided for this purpose.

The direct current derived from the output circuit of the electronic device 30 is impressed on the resistance 6 through a circuit which may be traced from the lower output terminal of the device 30 to a conductor 32, resistance 6, resistance 20, conductor 2|, recorder 22, indicator 23 and a conductor 33 to the upper output terminal of the device 30. The output terminals of the reconverter 30 are so related to the resistor 6 that the potential drop produced across the latter by the reconverter output current is of the proper polarity to cancel out the diiference in potential between that produced by thermocouple 2 and that created across resistance 6 by the resistor M.

It is noted that in order to maintain a. direct current output from the reconverter 30 it is necessary to maintain an alternating potential impressed on the input circuit of the electronic amplifier 26, and accordingly, to maintain some unbalance in the potentiometric network 5. The degree of unbalance of the potentiometric network 5 required to maintain a direct current output from the reconverter 30 throughout the range of the apparatus is extremely slight, however, and for all practical purposes the direct current output of the device 30 may be considered as actually rebalancing the potentiometric network 5, that is, reducing the unbalanced electromotive forces therein practically to zero.

The milliammeter recorder 22 and the milliammeter indicator 23 are provided in the output circuit of the reconverter 30 to measure the algegraic sum of the currents supplied to the resistor 6 from resistance II and from the reconverter 30. It will be apparent to thoseskilled in the art that the current flow through the resistance 8 from the resistance H and the reconverter 3|] which is required to rebalance the potentiometric network 5 will provide a measure of the temperature to which the thermocouple 2 is subjected. In addition, it will be clear that the milliammeter readings may be calibrated in terms of temperature to thereby provide a record or indication of the temperature within the interior of the furnace i to which the thermocouple 2 is subjected.

When the electromotive force developed by the thermocouple 2 is exactly that value which is maintained across resistance 6, the differential between the two opposed potentials will be zero, and therefore, the alternating current induced in the transformer secondary winding l0 and impressed on the input circuit of the amplifier 26 will also be zero. Under this condition the direct current output from the reconverter 30 will also be zero. In this condition of operation of the measuring apparatus it will be noted that the potentiometric network 5 is precisely balanced.

Upon a change in the electromotive force developed by the thermocouple 2 the state of precise balance of the potentiometric network 5 will be disturbed and a diii'erence in potential between the two opposed potentials will be produced. This difierence in potential is translated by the interrupter 1 and the transformer 9 into an alternating current of one phase or of opposite phase depending upon the polarity of the said difference in potential and thereby upon the direction of the change of the thermocouple electromotive force. This derived alternating electromotive force is amplified by the amplifier 26 and is translated by the reconverter 30 into a direct current of one polarity or of opposite po-. larity depending upon the phase of the derived alternating current. The output current from the reconverter 30 operates to produce a potential drop across the resistor B which is in opposition to the potentiometer unbalanced poten- 40 tials and thereby tends to restore the state of ance of the potentiometer network is required to maintain the output current from the reconverter 30. For all practical purposes, however, the extent of unbalance of the potentiometric network 5 required to maintain this output current from the device 30 is negligible and therefore the potentiometric network 5, in reality, is substantially rebalanced.

From the foregoing description it will be clear that the device from my present invention is a self balancing potentiometric network which is adapted to be continuously restored to a state of equilibrium by means responsive to potentiometric unbalance without requiring the use of physically movable apparatus such as galvanometers or similar devices for detecting the state of balance of the potentiometric network.- In addition, the potentiometric network is rebalanced or restored to a stabilized state without requiring the use of any physically movable apparatus whatever for this purpose. The device of my present invention includes a further advantageous feature in that the range of measurement of the apparatus may be adjusted by means of the knob n so as to enable a higher degree of accuracy to be attained over a portion or portions of the range of variation of the electromotive force developed by the thermocouple 2.

As pointed out hereinbei'ore, the interrupter or converter 1 illustrated schematically in Fig. l and in detail in Figs. 2, 3 and 4 operates to convert the potentiometric unbalanced direct currents int pulsating currents capable of being readily amplified. It will be understood that any suitable form of circuit interrupter may be employed but in order to illustrate an operative embodiment of the invention the interrupter shown in detail in Figs. 2, 3 and 4 may be utilized.

The interrupter 1 illustrated in Figs. 2, 3 and 4 is of the type shown and described in application Serial No. 421,176 and filed by Frederick W. Side on December 1, 1941. The interrupter 1 as shown in Figs. 2 and 3 is provided with a base 34 in which are mounted terminals 65, 36, 61, 38 and 36. A supporting plate 46 is secured to the base 64 by means of screws 4|. A stud 42 provided with a screw threaded extension 46 is secured to the lower end of the supporting plate 46 by means of a lock washer 44 and a nut 45. The free end of the stud 42 is bifurcated having spaced apart ends 46 and 41. Located between the spaced apart ends 46 and 41 are an insulating pad (not shown), a spring contact arm 46 carrying a contact 46, a resilient stop 56 and an insulating pad i, a vibrating reed 52 carrying a. contact 56, an insulating pad 54, a resilient stop 55, a spring contact arm 56 carrying a contact 51 and an insulating pad (not shown). These elements are all clamped between the spaced apart ends 46 and 41 by a bolt 58 and a nut 56. The spring contact arms 46 and 56 are provided with cars 66 and 61, respectively, which are electrically connected to the terminals 66 and 61, respectively. Th vibrating reed 52 is provided with an ear 62 which is electrically connected to the terminal 35. Riv eted to the supporting plate 46 are also studs 63 and 64 which carry adjustable stops in the form of screws 65 and 66 formed of insulating material. When the adjustable stops 65 and 66 have been adjusted as desired, they are clamped in place by means of screws 61 and 66, respectively. The spring contact arm 56 carrying the contact 51 through its own resiliency engages the resilient stop and the resilient stop 55 through its resiliency engages the adjustable stop 65. In like manner the spring contact arm 46 engages the resilient stop 56 which in turn engages the adjustable stop 66. By adjusting the adjustable stops 65 and 66 the positions of the contacts 51 and 46 may be independently adjusted with respect to the contact 53 carried by the vibrating reed 52.

A permanent magnet 66 is secured to the supporting plate 46 by screws 16 and 11. A coil 12 is held in place by a bracket 13 which in turn is secured in place by the screws 16 and 1|. The end of the vibrating reed 52 is disposed within the coil 12 and is provided with an armature 14 which is riveted to the vibrating reed 52 by rivets 15.

The coil 12 is energized with alternating current and acts on the armature 14 to vibrate the reed 52 at cycle per second when the alternating current supplied by the mains II and I2 is 60 cycle alternating current to cause the contact 56 to engage and disengage with the contacts 51 and 46. The permanent magnet 66 opcrates in conjunction with the coil 12 and the armature 14 in such a manner as to cause the armature 14 to vibrate in synchronism with the alternating current supplied by the mains II and I2. By adjusting the adjustable stops and 66, and hence the contacts 51 and 46, the wave form produced by the contacts 46, 56 and 51 may be adjusted to the desired value and shape. The contacts 46 and 51 are preferably so adlusted that when the contact 56 is in its stationary position it engages both contacts 46 and 51. This provides an overlapping action which compensates for wear oi the contacts, and also contributes to the elimination oi stray electrical effects on the operation or the apparatus. Due to this overlapping action also wear of the contacts does not materially alter the wave form produced by the contacts. By mounting the contacts 46 and 51 on the spring contact arms 46 and 56, respectively. 800d wiping contact is at all times provided between the contact 56 and the contacts 46 and 51. An electrical connection may desirably be provided between one of the screws 4| and ground so that the various parts of the interrupter are connected to ground to maintain the interrupter at ground potential. A cover not shown may also desirably be provided for enclosing the movable parts of the interrupter. Such a cover may be held in place on the base 64 by means of a rolled flange clamping the cover to the base. Such a cover will act to prevent dirt and corrosive atmosphere from aflecting the parts of the vibrator.

The interrupter 1 is essentially a polarized switching mechanism, the operating winding 12 and the permanent magnet 66 cooperating to vibrate the vibrating reed 52 at 60 cycles per second in synchronlsm with the 66 cycle alternating current supply. For purposes 01' explanm tion it may be assumed that the contact 51 is engaged by the contact 56 during the first half cycle of the alternating current supply when the voltage is positive and the second contact 46 is engaged by the contact 56 during the sec- 0nd half cycle when the voltage of the alternating current supply is negative. Accordingly, the contacts 56 and 51 engage when the volt age or the alternating current supply is positive and the contacts 56 and 46 enga e when the voltage of the alternating current supply is negative. when the vibrating reed 52 is stationary in its mid position both contacts 46 and 51 are engaged by the contact 56 so that when the vibrating reed is operated the contact 56 is always in engagement with one or the other 01' the contacts 46 and 51.

The manner of connection of the interrupter 1 to the primary winding 6 o! the transformer 6 is schematically shown in Fig. 4. By reference to Fig. 4 it will be noted that the contact 46 is connected to one end terminal oi. the trans former primary winding 6 and the contact 51 is connected to the other end terminal oi the winding 6. The contact 56 is connected through the reed 52 and the terminal 65 to the conductor 6 of the potentiometric circuit 6 and the conductor H of the potentiometrlc circuit is connected to a center tap on the transformer primary winding 6.

When the potentiometric network 5 is balanced no current flows in the potentiometric network and accordingly operation of the interrupter 1 is ineflective. When the electromotive force developed by the thermocouple 2 increases to unbalance the potmtiometric network I; in one direction, however, the unbalanced potentiometric direct currents flow in the direction from the transformer I to the interrupter l, and conversely when the electromotive force developed by the thermocouple 2 decreases to unbalance the pc tentiometric network I in the opposite direction the potentiometric unbalanced direct currents flow in the direction from the interrupter 1 to the transformer I.

More specifically upon unbalance f the potentiometric network I in the direction to ramder the conductor ll positive with respect to the potential of the conductor 1, current, during the first half cycle of the alternaL ing current supplied by lines If and II, will flow from the conductor I! to the midpoint of the transformer primary winding 8, through the lower half of the winding 8 to the contact 81, to contact 52 and the reed 52 to the conductor 1. During the second half cycle of the alternating current supply, current will flow from the conductor I! to the midpoint of the transformer primary winding 8, through the upper half of the winding 8 to contact 49, to contact 53 and through the vibrating reed $2 to the conductor 3. The pulsating direct current flow through the transformer primary winding 8, first through the lower half of the winding and then through the upper half of the winding, and acts through the core structure of the transformer 9, to introduce an alternating voltage in the transformer secondary winding I0 having a predetermined phase relatively to the phase of the alternating current supplied by the mains II and i2.

Upon unbalance of the potentiometric network in the opposite direction the conductor 3 will be rendered positive with respect to the conductor !5, and therefore, the current flows through the transformer primary winding 8 will alternately be from the lower end of the wind ing to the mid-point when the contacts 53 and 51 are in engagement and from the upper end of the winding to the mid-point when the contacts 49 and 53 are in engagement. These pulsating direct current flows through the transformer primary winding 8 are in the opposite direction from what they were when the potentiometric network was unbalanced in the opposite direction, and as a result, these ulsating direct current flows o erate through the core structure of the transformer 5 to induce an alternating voltage in the transformer secondary winding Hi which is of the opposite phase relatively to the voltage of the alternating current supply mains II and 12.

Accordingly, when the potentiometric network 5 is unbalanced in one direction an alternating voltage of on phase relatively to the voltage of the supply mains H and I2 is induced in the transformer secondary winding l0 and when the potentiometric network 5 is unbalanced in the opposite direction an alternating voltage of the opposite phase relatively to the voltage of the supply mains II and I2 is induced in the transformer secondary winding Hi. When the potentiometric network 5 is precisely balanced, there is no current fiow through the primary winding 8 of the transformer 9, and hence, the alternating voltage derived in the transformer secondary winding I 0 is zero.

The electronic amplifier 26 referred to is illustrated in detail in Fig. 5 and include an electronic valve (i which is preferably a heater type high mu triode having an anode, cathode and a control electrode. The input circuit of valve II is connected by conductors 11 and II to the terminals of the transformer secondary winding ll. Anode voltage is applied to the valve N from a rectifier 8| through a suitable filter 1| which is connected in circuit between the valve 10 and the rectifier II. The rectifier ii is a conventional full wave rectifier employing a rectifier valve ll including two heater type diodes in one envelope. current is supplied the heater filaments of the diodes from the low voltage winding 82 of a transformer 83 which also includes a line voltage primary winding it, a high voltage secondary winding I! and a second low voltage secondarywinding 88. The anode of one diode of valve II is connected to oneterminalofsecondarywindingflandthe anodooftheseconddiodeisconnectedtothe other terminal of the winding. The cathode of the diodes is comprised of the heater filament which is energized from the transformer secondarywindingllandiscommontobothofthe diodes. The cathode of the rectifier is connected to the positive input terminal of the filter l! and the negative input terminal of the latter is connccted to a center tap on the transformer secondary winding 85. The negative terminal of the filter may desirably be connected to ground potential. The filter I! may be of any suitable type and has its positive output terminal connected through resistances ll and 88 and a condenser 89 to its negative output terminal. Resistances I1 and II in conjunction with condensers 8! and provide additional filtering for the plate current supply for the triodes of the first and second stages of the amplifier.

Energizing cunent is supplied the heater filament of the electronic valve 15 from the low voltage secondary winding 8i through conductors not shown in order not to confuse the drawings. The transformer secondary winding 86 further supplies energizing current to the heater filaments of electronic valves 80 and ii. of anode current through the electronic valve l6 is normally maintained at a mean value by virtue of the action of a cathode biasing resistance 82 and a condenser 81 connected in shunt to the resistance '2.

When an alternating potential appears across the terminals of the transformer secondary winding II, the conductivity of the valve '16 is alternately decreased and increased resulting in a pulsating potential drop appearing across a resistance 94 connected in the output circuit of the valve 18. The output circuit for the valve 16 may be traced from the positive output terminal of the filter "I! through resistors 81, 88 and 84 to the anode of valve "ii, the cathode thereof, and the cathode biasing resistance 92 and shunt condenser to the negative terminal of the filter.

A resistance-capacity coupling is provided between the output circuit of the tube 76 and the input circuit of tube by a condenser 95 and a resistance 96. The valve 98 is a heater type valve and includes anode, cathode, and control elements. Anode voltage is supplied the valve II from the positive terminal of the filter through the resistance ll, aresistance 91, the anode of valve 98, the cathode, and a cathode biasing resistance 98 which is shunted by a condenser 99 to the negative terminal of the filter.

The output circuit of the valve 80 is resistancecapacity coupled by a condenser I00 and a resistance Ill to the input circuit of the electronic valve 9! and the output circuit of the latter is The flow.

connected to the terminals of the primary winding 21 of the transformer 20. The output circuit of the valve 9I may be traced from the positive terminal of the filter 19 to a conductor I02, the transformer primary winding 21 as may be seen by referring to Fig. 1, a conductor I03 to the anode of valve SI, the cathode of valve SI, and a biasing resistance I04 shunted by a condenser I05 to the negative terminal of the filter.

Thus, when an alternating voltage of one phase or of opp site phase is impressed upon the input circuit of the electronic valve 16 an alternating voltage of corresponding phase is produced in the secondary winding 29 of the transformer 28.

The reconverter 30 which is employed to transform the alternating current output of one phase or of opposite phase from the electronic amplifler 26 into a direct current of one polarity or of opposite polarity is illustrated in detail in Fig. 6. As shown in Fig. 6, the reconverter 30 comprises a pair of electronic valves I06 and I01 which are heater type triodes having anode, cathode, and control electrode elements and the output circuits of which are energized in parallel from the secondary winding I of a transformer I09 having a line voltage primary -vinding I I0 which is connected to alternating current supply means II and I2, and a low voltage secondary winding I I I. The cathodes of the triodes I06 and I01 are connected together and are also connected through a biasing resistance II 2 to the left end terminal of the transformer secondary winding I08. The anode of the valve I06 is connected through a resistance II3 to the right end terminal of the transformer secondary winding I08 and the anode of the valve I01 is connected to the latter terminal through a resistance II. Thus, the output circuits of the electronic valves I06 and I01 are energized in phase with alternating current of the frequency of the supply lines II and I2.

The input circuit of the electronic valve I05 includes the cathode blming resistance H2, and a resistance II5 which is connected between the negative terminal of the resistance H2 and the control electrode, and the input circuit of the valve I01 includes the resistance H2 and a resistance H6 which is connected between the negative terminal of the resistance I I2 and the control electrode of valve I01, The alternating current output from the transformer secondary winding 28 is impressed on the resistances H5 and I I6 in series by means of conductors I I1 and I I8 and is either in phase with the alternating voltage applied to the output circuits of the valves I06 and I01 or is displaced 180 therewith. Such phase relation is readily obtained by virtue of the fact that the electronic amplifier 26 and the reconverter 30 are both energized from the same alternating current supply source.

One output terminal of the reconverter 30 is connected from the point of engagement of a contact 3' which is in engagement with and is slideable along the resistance II3 to the conductor 33 and the other output terminal of the reconverter 30 is connected from the point of engagement of the resistance III and the anode of valve I01. When the alternating voltage impressed across the resistances H5 and I I6 by the transformer secondary winding is zero the control electrodes of both of the valves I06 and I01 will be at the same potential, namely at the potential at the negative end of the resistance H2, and consequently the valves I06 and I01 will be equally conductive. The resistances Ill and III are or the same value, and therefore, the anodes of valves I00 and I01 will then be at the same potential. The contact I I3 may be manually ad- Justed along the resistance III in unison with adjustments of the contact I Salong the slide wire resistance It in such manner that when the anodes of valves I06 and I01 are at equal potential, the potential between the contact II 3' and theanode of valve I 01 is equal to the potential tappe oi! the slide wire resistance I4 by contact I6. Since these potentials are connected in opposition, no current is drawn from the reconverter 30 through conductors l2 and 23. Under this condition the current which flows through resistors 6 and 20 and the milliammeter instruments 22 and 23 is all of the current drawn from the resistance H and through resistance 25. Stated differently, the current through the resistance 25 is then the same current which flows through resistances 6 and 20 and the instruments 22 and 23. In accordance with this embodiment of the present invention, it is contemplated that this current iiow through resistance 6 produces a potential drop across the latter of the required value to exactly balance the normal operating value of thermocouple electromotive force.

When an alternating current of one phase relatively to the phase of the current supplied by the mains II and I2 is impressed across the re-- sistances H5 and H6 as a result of a reduction in the electromotive force produced by the thermocouple 2, however, the control electrode of valve I06 will be rendered less negative during the half cycle when the anode of that valve is positive while the control electrode oi the other valve will be rendered more negative during the half cycle when its associated anode is positive. Accordingly, the valve I06 will then be rendered more conductive than the valve I01. Under this condition the potential drop across the valve I06 is less than the potential drop across valve I01, whereby, the output terminal of the reconverter which is connected to the conductor 33 will be rendered somewhat less positive with respect to the output terminal connected to the conductor 32. This will divert part of the current, which previously flowed through resistances 6 and 20 and instruments 22 and 23 from resistance II, through the conductor 33, reconverter 30, conductor 32 and resistance 25 to resistance II. Therefore, the effect 01' the application or an alternating current of the phase referred to the input circuit of valves I06 and I01 is to reduce the current flow through resistances 6 and 20 and instruments 22 and 23. This reduced current flow through resistance 6 creates a reduced potential drop across the resistance 6 of the required value to substantially rebalance the potentiometer 5.

When an alternating voltage of the opposite phase is impressed across the resistances IIS and H6 as a result of an increase in the thermocouple electromotive force, the control electrode of the valve I01 will be rendered less negative with respect to its associated anode during the half cycle when the latter is positive while the potential of the control electrode 0! the valve I06 will be rendered more negative relatively to its associated anode when the latter is positive. Consequently, the valve I01 for this condition will be rendered more conductive than the valve I00. The potential drop across the valve I01 is then less than that across valve I 00 and as a resul the output terminal of the reconverter connected to the conductor II willbe rendered more positive with respect to the output terminal connected to the conductor 32." As a result of this increased potential between conductors I3 and 32, current will now through resistances l and 24 and instruments 22 and 23 from the reconverter 30 which will supplement that through these elements from resistance I4. The effect oi this increasedcurrent flow through resistance 4 is to produce an increased potential drop across the latter as required'to substantially rebalance the potentiometer 5.

From the foregoing description it will be apparent that the current for maintaining the potentiometer Sin a balanced condition is wholly derived from the resistance I4 when the thermocouple electromotive force is its normal operating value, and that no current is then drawn from the reconverter 80 for this purpose. Upon unbalance of the potentiometer I in one direction the reconverter 30 operates to divert part of the current fiow through resistance 6 t eifect potentiometer rebalance, and upon unbalance of the potentiometer in the opposite direction, the reconverter operates to supply additional current to resistance 8 to efiect potentiometer rebalance. Such additional current need be only a small part of the total current required in resistance 6 to balance the potentiometer.

The condenser 3i is provided for the purpose of soothing out the pulsations or ripple in the direct current fiow derived from the output circuit of the reconverter 30. It will be clear that additional stages of filtering may be employed, if

desired.

It will be noted that the condition of drawing minimum current from the reconverter 30 may be adjusted to correspond to different normal operating values of thermocouple electromotive force as desired, by moving the contact 16 to the position along slidewire resistance I4: For the most perfect attainment of this condition, from a theoretical standpoint, the contact 3' should be adjusted along resistance Hi to a position corresponding to the position of the contact I8 along resistance I4. To this end, the contacts I6 and H3 may desirably be mechanically connected so that their adjustment may be enected simultaneously by a, single manipulation. Such refinement is not ordinarily required in many industrial applications, however, and therefore, the feature of adjustabilityo! contact II3 along resistance II3 may be dispensed with. In addition, it is ordinarily sufilcient to permanently connect the conductor 33 to the anode of valve I06 when the resistances H3 and H4 are of equal magnitude.

As will be understood by those skilled in the art, the contact II 3' may be adjusted along the slidewire resistance I I3 as required to derive a potential between conductors 32 and 33, when no signal is impressed on the input circuits of valves I08 and it'll, oi the proper magnitude to create that potential drop across resistance 6 which is required to balance the thermocouple electromotive force whereby the elements I3, I4, I5, I6, II, I8, 24 and 25 may be dispensed with. In such modification, the conductor 32 from the reconverter may be connected through resistances 4 and 20 and instruments 22 and 23 to the conductor 33. The application of a. signal of one phase or of opposite to the input circuits of valves I08 and I0! in response to potentiometer unbalance in one direction or the other will operate to vary the current flow from the reconverter as required to rebalance the potentiometer.

In Fig. 'i I have illustrated more or less diagrammatically a modification oi the measuring system disclosed in Figs. 1-6 in which the rectifier I 8 and the resistance I4 and condenser I! have been dispensed with and*the direct current potential derived by means of those elements is obtained from the full wave rectifier 84 which supplies the direct current plate voltages for the electronic amplifier 24. Fig. 7, in addition, illustrates in detail one form of filter I! which may be utilized in the electronic amplifier 28 and shown only schematically in Fig. 5. Fig. '7 also illustrates a different form of reconverter which may be utilized in lieu of the reconverter 30 of the arrangement of Fig. 1.

Specifically, the thermocouple circuit in Fig. 'I may be traced from one terminal of the thermocouple 2, through conductor 4. to the resistance 6, to conductor I9, and to the mid-point of the transformer primary winding 8 the opposite ends of which are adapted to be alternately connected to the other terminal of the thermocouple by means of the interrupter or converter I and the conductor 3. The electromotive force produced by thermocouple 2 is opposed to the potential drop produced across the resistance 6 partly by a fiow of direct current derived from the full wave rectifier through the filter 19 and partly by the fiow of direct current derived from the reconverter. The circuit from whichthe first mentioned direct current is derived may be traced from the positive terminal of the filter 15 through a conductor I20 in which a resistance I2I is inserted, through resistance H9, and through a conductor I22 to the negative terminal of the filter. It will be noted that this fiow of direct current from the rectifier 80 and filter I9 operates to render the upper end of resistance H9, as seen in the drawing, positive with respect to the lower end of the resistance. The circuit from which the second mentioned direct current is derived includes a resistance I24 through which current from the reconverter is passed. The direction of such current flow through resistance I24 is such as to render the upper terminal of the latter positive.

The upper ends of the resistances H9 and I24 are connected to each other through resistance 6 and instruments 22 and 23 and the lower ends of resistances IIS and I24 are connected by a conductor I23. Since the upper ends of both of the resistances H9 and I24 are positive, a current will fiow through resistance 8, and instruments 22 and 23 in one direction or the other depending upon whether the potential drop across resistance I24 is greater or less than that across resistance 9. The electromotive force generated by thermocouple 2 is opposed to the potential drop produced across resistance 5 by such current fiow therethrough in a; potentiometer circult which has been indicated generally by the reference character 5'. When the-potential drop across resistance 6 is equal to the thermocouple electromotive force, the potentiometer will be precisely balanced.

The rectifier 80 comprises an electronic valve 8i which is a full wave rectifier valve and has its two anodes connected to opposite terminals of the secondary winding I25 of a transformer I26. The transformer I26 also includes a line voltage primary winding I21 and a high voltage secon dary winding I28. The rectifier valve 8! may be or the type in which the heater filament comprises the cathode or may be of the type in hich an indirectly heated cathode is provided. It will be apparent that the rectifier valve 2| may also be of the indirectly heated cathode type, it desired The filter I9 includes resistance and capacitive components which are cooperatively related to each other so as to smooth out the ripple in the direct current output of the rectifier valve 9|. As shown, more stages of filtering may be provided for producing the direct current potential drop across the resistor I19 than are provided for energizing the anode circuits of the electronic amplifier 25 in order to insure the establishment of a smooth direct current potential tree from ripple across the resistance 9.

The electronic amplifier 26 of Fig. 7 may be identical to the electronic amplifier 29 shown in Fig. 5. The output circuit of the electronic valve 9| is connected through conductors I02 and I93 to the terminals of the primary winding 21 of the transformer 28 which also includes the secondary winding 29. The terminals of the transformer secondary winding 29 are connected to the input circuit of a single electronic valve I29 which is utilized in the Fig. 7 arrangement in lieu of the two electronic valves I06 and II" employed in the reconverter 30 of Fig. 6.

The electronic valve I29 is a heater type triode having an anode, a cathode and a control electrode. The filament heater oi the electronic valve I29 may be energized from the same source .of alternating current which supplies energizing current to the heater filaments of the electronic valves I5, 90 and 9I or, if desired, a, separate source of alternating current may be provided for energizing the heater filament of the valv I29. The input circuit of the electronic valve I29 may be traced from the cathode thereof through the resistances I32 and I24 to the conductors I23 and I8 to the lower terminal of the transformer secondary winding 29 and through the latter to the control electrode of valve I29. The resistors I32 and I24 operate as cathode biasing resistors and tend to place a negative bias on the control electrode. The anode circuit of the valve I29 may be traced from the lower terminal of the transformer secondary winding I28 through a conductor I3I to the anode of valve I29, the cathode, resistance I32, resistance I24 which is shunted by resistances 6 and I I9 and meters 22 and 23 and conductor I33 to the upper terminal of the transformer secondary winding I28. A condenser I30 connected between the cathode and the lower terminal of the transformer secondary winding I29 is provided for smoothing out the ripple in the valve output current.

When the resultant of the potential drop Droduced across the resistance Ii and the electromotive force of the thermocouple 2 is zero, no alternating voltage will be induced in the transformer secondary winding 29 and in this condition of operation the electronic valve I29 will conduct a predetermined amount of current depending upon the self biasing action of the resistors I32 and I24 on the control electrode. The biasing resistors may desirably be so adjusted that the current conducted with no impressed signal is intermediate its normal range of variation.

Upon unbalance in the potentiometric network produced by a change in electromotive force generated by the thermocouple 2, the resultant of the potential drop produced across the resistances I and the potential will be translatedbytheintermptcrl andthctransformer 9 into an alternating voltage across the terminals of the transformer secondary winding III. This alternating voltage will be amplified by the electronic amplifier 23 into In alternating voltage oi corresponding voltage but oi. increased magnitude across the terminals or the transformer secondary winding 22. The alternating voltage so induced in the transformer secondary winding 22 is applied to the input circuit 0! the electronic valve I22 and operntu to alternately oppose and assist the nentive bias maintained on the control electrode by the biasing resistors I32 and I24.

When the direction or potentiunetric unbalance is such as to cause the alternating voltage applied to the control electrode of valve I22 to0ppose the negative biasmaintamed on the control electrode by the cathode biasing resistors during the half cycle of the alternating voltage supply when the anode of valve I2! is positive, the conductivity of the electronic valve I29 will be increased to an extent depending upon the magnitude or the alternating voltage induced in the winding 29. The flow 0t anode current through the resistance I24 from the valve I29 operates to produce a potential drop across this resistance which is in the proper direction to reduce the unbalanced electromoflvc forces in. the potentiometric network 3'. The potentiometric electromotive forces in the potentiometric network 5' will not be exactly reduced to zero since some slight unbalance isrequired to maintain the increased current flow from the electronic valve I 29 required to be delivered to the resistance I24 for rebalnncing the potentiometer. The extent to which the potentiometric network 5' must be unbalanced (or this purpose is extremely slight, however, and therefore, the potentiometric network I for all practical purposes may be regarded as being exactly balanced. The resultant flow of current through the instru ments 22 and 23 and the 6 for this new value oi potential drop across resistance I24 will operate to provide a measure of the extent to which the thermocouple electrornotive has changed and thereby to provide a measure of the temperature to which the thermocouple 2 is subjected. It is noted that current flow through resistance 6 and instnnnents 22 and 22 may either increase or decrease upon such change in potential drop across resistance I24 depending upon whether the potential drop across resistance I24 originally was greater or less than the potential drop across resistance II9.

Upon unbalance of the potentiometric network 3' in the opposite an alternating voltage of opposite phase will be induced in the transformer secondary winding 29. This alternating voltage will operate to increase the negative bias on the control electrode of valve I29 during the half cycle when the latter is conductive, and therefore, to effect a reduction in the anode current fiow from the valve I29 through the resistance I24. The resulting decrease in potential drop across resistance I24 will operate to efi'ect a change in the current flow through resistance I 01' the proper amount to restore the state of balance of the potentiometric network 5'. The meters 22 and 23 provide an indication of the change in the current flow through resistance 4 and thereby provide a measure of the temper ture to which the thermocouple 2 is subjected.

InFig.8Ihaveillustratedamodiflcatlono2 the secondary winding I 0! the transformer 9 the arrangement disclosed in Fig. 7 in which the transformer 20 oi! Fig. 7 has been dispensed with and in which the reconverter tube I29 is controlled by direct electrically conductive connections t the resistance I04 of amplifier 26. In addition, Fig. 8 discloses the use of a separate transformer I34 having a, line voltage primary winding I35 and a secondary winding I36 for energizing the output circuit of the valve I29 but it will be understood that, if desired, a winding I28 may be provided on the transformer I26 for this purpose as in the Fig. 7 arrangement.

In Fig. 8 a negative bias is'normally maintained on the control electrode of valve I29 which is the resultant the potential drops across resistors I32, I24 and I04. The potential drop across resistance. I04 is opposed to the potential drops across resistances I32 and I I24. A condenser I30 is also provided for smoothing out the ripple in the valve output current so as to cause that current to be substantially smooth and steady. The input circuit of the valve I29 may be traced from the cathode-through the biasing resistors I32 and I24 to the conductor I0, resistor I04 and a current limiting resistor I31 to the control electrode of valve I29. In Fig. 8 the resistor I 04 of the amplifier 26 is not shunted by a condenser although a condenser may be connected in shunt to this resistor if so desired. The opera tion of the arrangement of Fig. 8 in other respects is identical to that of Fig. 7 and therefore further explaination is not believed necessary.

In Fig. 9 I have illustrated, more or less diagrammatically, another modification of the measuring system shown in Fig. 7 in which the output stage of the amplifier 26 is energized with alternating current instead of direct current and in which the 'reconverter is of the type shown in Fig. 7 but instead of being energized with alternating current it is energized with direct cur-- rent. Reconverter valve I29 in Fig. 9 is supplied with anode voltage through a circuit which may be traced from the positive terminal of the filter I9 to the anode of valve I29, the cathode thereof, cathode biasing resistances I32 and I24, the latter of which is shunted by resistances 6 and H9 and meters 22 and 23, and through conductor I22 to the negative terminal of the filter. The input circuit of the valve I29 may be traced from the cathode through resistances I32 and I24 and the shunt circuit of the latter, a conductor I38, resistance I04 shunted by condenser I05, and resistance I3'I to the control electrode. A negative bias potential is applied to the control electrode of valve I29 by the biasing resistors I32 and I24, but this biasing potential is adapted to be reduced in magnitude by the potential drop produced across the resistance I04 by the flow of current from the electronic valve 9| therethrough.

The output circuit of the'electroriic valve 9| receives energizing current from the secondary winding I39 of a transformer I40 having a line voltage primary winding I4I The output circuit of valve 9| may be traced from the left end terminal of the winding I39 to the anode of valve 9 I, the cathode thereof, the biasing resistance I 04 and condenser I connected in shunt therewith, conductor I38, and a conductor I42 t the right end terminal of the winding I39. The input circuit of the valve 9| is controlled by the output circuit of the valve 90 to which it is resistancecapacity coupled by condenser I00 and resistance IOI.

When the potentiometric network 5 is exactly balanced, no alternating voltage is induced in 7 and under this condition the current conducted by the electronic valve 0| is maintained at a predetermined value, preferably intermediate its normal range of variation, by virtue of the action of the cathode biasing resistance I04 and the condenser I05 which are properly chosen to attain this end. Accordingly, the potential drop across the resistance I04 and condenser I05 will then be a predetermined value. The potential impressed on the control electrode of the valve I29 will then be negative by the proper amount to maintain the current conducted by valve I29 intermediate its normal range of variation.

Upon unbalance of the potentiometric network' 5, tne alternating electromotive force induced in the transiormer secondary winding I0 will be amplified by the electronic valves 16 and and applied to the input circuit of the valve 9|. When this alternating electromotive force is of the proper phase to drive the control electrode of valve 9| less negative during the half cycles of the alternating voltage supplied by the transformer secondary winding I39 when the anode of valve 91 is positive, the conductivity of the valve 9| will be increased with the result that the potential drop across .the resistance I04 and condenser I05 will be correspondingly increased. The increased potential drop across resistance I04 and condenser I05 will operate to reduce the negative bias on the control electrode of valve I29 and accordingly increase the conductivity of the valve I29. he extent to which the conductlvity of valve I29 is increased is dependent upon the increase in potential drop across the resistance I04 and condenser I05 and thereby by upon the extent to which the potentiometric network 5 has been unbalanced.

The change in the output current from the electronic valve I29 produces a change in the potential drop across the resistance 6 in the proper direction to rebalance the potentiometric net work 5'. The magnitude of the current through the resistance 6 required to iebalance the potentiometric network 5 is indicated by the meters 22 and 23 and thus provides a measure of the temperature to which the thermocouple 2 is subiected.

In Fig. 10 I have illustrated a modification of the arrangement shown in Fig. 9 wherein the transformer I40 for energizing the anode circuit of the electronic valve 9| has been' dispensed with and alternating current for this purpose is obtainedby means of a winding I28 provided on the transformer I26. Fig. 10 also distinguishes from the arrangement disclosed in Fig. 9 in that the resistances H9 and I24 have been dispensed with whereby the electromotive force developed by the thermocouple 2 is opposed only to the electromotive force produced across resistance 6 by the flow of output current from the electronic valve I29. The circuit through which those electromotive forces are opposed has been indicated generally by the reference numeral 5" and also comprises a potentiometric measuring network. This modification further distinguishes from the preceding arrangements in that it is contemplated to unbalance the potentiometric network 5"- in one direction only. In other words the potentiometric network 5" will be precisely balanced only when the electromotive force produced by thermocouple is a maximum value and the output current from valve I29 is also a maximum value.

The current flow from the thermocouple 2 through the resistance I may be traced from the lower terminal of the thermocouple 2 through conductor 4 to resistor 6, the transformer primary winding 8, interrupter l, and conductor 2 to the upper terminal of the thermocouple. The output circuit of the reconverter valve I25 may be traced from the positive terminal or the filter 19 to the anode circuit of the valve I20, through indicator 23, recorder 22, resistor 5 and conductor I22 to the negative terminal of the filter. The thermocouple 2 is so connected to the circuit that its electromotive force is opposed to the potentiad drop produced across the resistance 6 by the flow of current from the reconverter I29.

The input circuit of the electronic valve I2! is controlled i accordance with the potential drqp produced across a resistance I45 which is shunted by a condenser I44 and is connected in the output circuit oi the valve 8|. The resistance I42 and condenser I44 are connected between the control electrode and the cathode of the valve I29.

With this arrangement, when the potentiometric network 5" is in a balanced condition, the conductivity of electronic valve 9| will be at its lowest value by virtue of the action of the cathode biasing resistance I04 and a condenser I55 which are chosen as to give this result, and consequently, the difference in potential between cathode and control electrode oi the valve I29 will be at a minimum. Accordingly, the valve I2! will be fully conductive. The potential drop produced across the resistance 5 by the flow of output current from the valve I29 will then exactly balance the maximum electromotive force produced by thermocouple 2.

Upon unbalance of the potentiometric network following a reduction in the thermocouple electromotive force, an alternating voltage will be applied to the input circuit of the electronic valve M. This alternating voltage will be or the proper polarity to render the control electrode of valve 9i less negative during the hall cycles that its anode is positive. The conductivity oi. the electronic valve 9| will then be increased whereby a potential drop will be produced across the resistance I43 in the proper direction to drive the control electrode of valve I28 negative with respect to the potential of its cathode. The conductivity of the valve I29 will then be decreased with the result that a corresponding decrease in the potential drop across resistance 6 by the flow of output current from the tube I25 will be eifected. This is in the proper direction to rebalance the potentiometric network 5", and such rebalance will be eflected to a high degree of precision notwithstanding the fact that some slight unbalance is required to maintain the output current from the valve I29 at this decreased value. The flow of current from the valve I2! through the indicator 23 and recorder 22 provide a measure of the temperature to which the thermocouple 2 is subjected.

In Fig. 11 I have illustrated, more or less diagrammatically, another modification of the arrangement of Fig. '7 wherein a reconverter having a pair of output tubes I45 and I46, which are energized in opposite phase, is provided to give full wave excitation to the resistance I24 connected in the cathode return circuit of the output tubes. Tubes I45 and I45 are heater type triode and include anode, cathode, control electrode and'heater filament elements. In this modification, as well as in the arrangement of Fig. 7, a

potential drop 01' predetermined magnitude is produced across the resistor III by a direct current flow derived from the rectifier 40. The conductors 4 and II leading from the thermocouple 2 and the interrupter 1 are connected across the terminals oi the resistor 5. The resistors 5, Ill, and I24 are connected in a series circuit in which a recorder 22 is also connected for recording the variations of temperature to which the thermocouple 2 is subjected. If desired, an indicator 23 may also be connected in this series circuit.

The output circuits oi the electronic valves I45 and I4 receive energizing current from the transformer secondary winding I25, which transformer secondary winding also supplies energising current to the full wave rectifier valve II. Specifically, one end terminal 0! the winding I25 is connected to the anode or valve I45 and the other end terminal of the winding I25 is connected to the anode of valve I48. The cathodes of the valves I45 and I4! are connected together and are connected through a resistance I41 and the resistance I24 to a point intermediate the ends 01' the winding I25.

The input circuits of the valves I45 and I48 are arranged to be controlled by the output stage of the electronic amplifier 26. Thus, the output circuit of the electronic valve SI of amplifier 25 is connected to the primary winding I44 0! a transformer I45 having a center tapped secondary winding I50. One end of the winding I50 is connected ,to the control electrode of valve I45 and the other end of the winding is connected to the control electrode of valve I45. The center tap o! the winding I50 is connected through the resistances I24 and I41 to the cathodes oi the valves I45 and I45.

With this arrangement, when the potentiometric network 5 is balanced, no alternating voltage is induced in the secondary winding I50 and under this condition the potentials of the control electrodes of valves I45 and I45 are maintained equal to each other and somewhat negative with respect to the potentials of their associated cathodes by virtue of the action of resistances I24 and I41. The flow of cathode current from the valves I45 and I45 through the resistances I24 and I41 produce a potential drop across these resistances in the direction to tend to apply a negative bias to the control electrodes of the valves. The potential drop produced across the resistance I24 during this condition of operation is precisely that value required to eilect a balance between the potential drop across the resistance 6 and the thermocouple electromotive force.

Upon unbalance or the potentiometric network 5' in one direction, an alternating voltage will be induced in the transformer winding I50 0! the proper phase to cause the control electrode of valve I45 to become less negative during the half cycle of the voltage supply when the anode of the valve I45 is positive, and thus operates to increase the conductivity of valve I45. During this half cycle the control electrode of valve I46 will be driven more negative during the half cycle when the anode of that valve is also negative and the valve is non-conductive. During th next half cycle of the alternating voltage supply the potential of the control electrode of valve I45 will be rendered more negative but during this half cycle valve I45 will be non-conductive since the anode is then negative. In this half cycle the valve I46 will be conductive and the potential of the control electrode of valve I48 will be driven less negative. Consequently, the conductivity of I24 is connected in the common return circuit of the two valves I45 and I46 an increase in the potential drop across the resistance I24 will be produced inasmuch as the half wave pulses of cur-- rent from the valves I46 and I46 during alternate half cycles are increased.

' Upon unbalancing of the potentiometric network in the opposite direction, the alternating voltage produced in the transformer secondary winding I50 is of the proper phase to render the control electrodes of valves I45 and I46 more negative during the alternate half cycle when these valves are conductive. Accordingly, the pulses of output current from the valves I45 and I46 are decreased whereby a corresponding decrease in potential drop across resistance I24 is eifected. This decrease in potential drop across resistance I24 is of the proper amount'to effect rebalance of the potentiometric network 5'.

In Fig. 12 I have illustrated a still further modiflcation of the arrangement of Fig. 1 wherein suitable provisions aremade for maintaining the temperature within the furnace I constant to a. high degree of accuracy notwithstanding variations in the load within the furnace or changes in the heating value of the fuel supplied for heating the furnace. It is known in the art that on a change in an operating condition such, for example, as a change in the furnace load or on a furnace I at the desired value the total current through resistance I52 is a predetermined value and upon change in the furnace temperature from the desired value the change in current flow through resistance I52 which tends to be produced as a result of the reconverter 30 diverting current from the resistance 6 or supplement ing the current flow through the latter is at least partiall compensated temporarily by a change in the current flow to the resistance I52 from the control networkreferred to. The manner in which this eifect'is accomplished and the operation when the initial corrective adjustment of the change in the B. t. u. content of the fuel supertia the change in the operating condition will exist for some time before it results in a temperature change which may be detected by the apparatus employed to maintain the furnace tempera ture at the desired value. When thereafter a correction in the amount of fuel supplied to the furnace is made to restore the furnace temperature to its desired value, such correction is not effective to immediately restore the desired condition. This lag is also due to the heat inertia of the furnace. In addition, if a sufficiently large corrective adjustment is made to restore the furnace temperature to the desired value within a relatively short time, and is maintained until the desired value is reached, the furnace temperature will tend to overshoot that value and subsequent corrective adjustments will result in oscillation of the furnace temperature about the desired v value.

Suitable control provisions, now to be described, are provided in the arrangement of Fig. 12 to prevent such hunting or oscillating and to effect control or regulation of the furnace temperature Fig. 1 and described in connection with that figure. The measuring system of Fig. 12 differs from that shown in Fig. 1 only in that a fixed resistance I52 is connected in circuit with the recorder 22 and indicator 23. Current flows through the resistance I52 from the resistance I4, from the reconverter and from a control network to be described. In the normal operation of the apparatus with the temperature of fuel valve I5I is insufllcient to restore the furnace temperature to the desired value is exlained hereinafter.

The initial effect of any change in the potential drop across resistance I52 is to produce a change in potential applied to an electrical control network I53 and thereby to vary the energization of a solenoid I54 as required to effect a corrective adjustment of the fuel valve I5I and also to effect a corresponding network neutralizing or follow-up adjustment of the potential drop across the resistance I52 to restore that potential to its original value. The electrical network I53 in cludes condensers I55, I56and I56 and resistances I55a and I56a. The components I55 and I55a are connected in parallel as are also the components I56 and I56a. These parallel circuits are connected in series with each other and with condenser I56 between the terminals of resistance I52.

Changes in the potential applied to the electrical control network I53 are arranged to be detected by a pair of electrical valves I51 and I58. Valve I51 is a heater type triode and includes anode, cathode, control electrode and heater filament elements. Valve I58 may also be of the same type. The input circuit of valve 351 may be traced from the cathode to a battery I59 and the parallel connected condenser I55 and the resistance I55a to the control electrode. The input circuit of the valve I58 may be traced from the cathode to a battery I60, an adjustable contact I6I for tapping off a desired portion of the voltage of the battery I60, a conductor I62, battery I59 and conductor I63, resistance I52 and a conductor I64 to the control electrode.

The output circuit of the valve I51 is energized by a battery I65 through a circuit which may be traced from the positive terminals of the battery to the anode of valve I51,the cathode, battery I59, conductor I63, resistance I52, and conductor I64 to the negative terminal of the battery. It is the flow of output current from valve I51 through resistance I52 which is utilized in this form of my invention to at least partiall compensate temporarily for any change in current flow through the resistance I52. The output circuit of the valve I58 is energized by a battery I60 through a circuit which may be traced from the positive terminal of the battery through a conductor I66, in which the solenoid I54 is connected, to the anode of valve I58, and the cathode of the valve to the negative terminal of the battery I60.

Battery I59 which is connected in the input circuit of the valve I51 tends to maintain a potential on the control electrode thereof which is negative with respect to the potential of the cathode, and similarly the portion of the battery I60 which is tapped oil" by the contact I6I tends to maintain a positive potential on the control electrode of the valve I58 relatively to the potential of its associated cathode.

When the furnace temperature has been stabilized at the desired value for an appreciable period of time, the potential drop across the resistance I52 will be at a steady predetermined value depending upon the adjustment of the contact I6 along the slidewire resistance I4, and under this condition the potential of the control electrode of valve I51 will be that maintained thereon by the battery I59 alone. This is the case because of the action of resistance I55a in operating after a period of time to discharge from condenser I55 any charge that may previously have existed thereon. The potential maintained on the control electrode of valve I58 will then be that maintained thereon by the resultant of the portion of battery I60 tapped oil, battery I58 and the potential drop across resistance I52.

The initial eiIect of any change in the furnace temperature will be to produce a change in the state of balance of the potentiometric network and thereby to cause a change in the flow of current from the output circuit of the reconverter 30 through the circuit including resistor I52 for effecting a rebalancing adjustment of the potentiometric network. The resulting change in potential drop produced across the resistance I52 operates to change the potential on the control electrode of valve I58 and also simultaneously operates to change the potential on the control electrode of valve I51. The change in potential across resistance I52 is eflective immediately to cause a change in the conductivity of valve I58 and thereby to change the energization of solenoid I54. The sense of this change in the energization of solenoid I54 is that required to effect a corrective adjustment of valve I5I 1n the direction to restore the furnace temperature to the desired value.

Due to the action of the parallel connected condenser I55 and resistance I56a, however, the change in potential across resistance I52 is not effective immediately to cause a complete change in the potential of the control electrode of valve [51 relatively to the potential of the cathode. The condenser I56 and resistance I56a operate to temporarily delay the application of the full change in the potenital across resistance I52 to the control electrode of valve I51 and thereby operate to temporarily delay the full compensating action of change in the output current from valve I51 upon the current flow through resistance I52. As is explained hereinafter this temporary delay operates to introduce an initial magnetification in the adjustment of valve I5I and is desirably of a duration proportioned to the overall characteristics of the furnace I, the load therein and the fuel utilized whereby the initial magnified adjustment of valve I5I tends to effect quick restoration of the furnace temperature to the desired value. Such proportion can be obtained by properly choosing condenser I56 and resistance I55a in relation to the values of the other circuit components.

The temporary delay in transferring a maximum potential change across resistance I52 to the control electrode of valve I51 introduced by condenser I 56 and resistance I561: is obtained by virtue of a transient eifect introduced by the latter elements, which transient effect tends to minimize the change in the potential of the control electrode of valve I51 in response to a given change in potential across resistance I52. As

this transient eflect gradually dies out, the potential change applied to the control electrode of valve I51 gradually approaches a condition of proportionality with the change in potential across resistance I52 and, therefore, the output current flow from valve I51 undergoes a change as required to compensate at least partially for the change in potential across resistance I52.

More specifically, immediately upon change in the furnace temperature from the desired value and a consequent change in potential drop across resistance I52, the potential across condenser I55 changes an amount depending uponthe capacity of condenser I55 in relation to the capacities of condensers I55 and I55. This change in potential on condenser I55 is applied to the input circuit of valve I51 and operates to cause a corresponding change in output current from valve I51. This change in output current is in the direction to compensate for the change in current flow through resistance I52 but is of such small magnitude as to only partly compensate therefor.

As that part of the potential change across resistance I52 which has been assumed by condenser I55 dies out as a result of being dissipated through resistance I55a, the potential applied to the control electrode of valve I51 is further increased in the same direction, and accordingly, an additional change in the output current of valve I51 is produced to further and more completely compensate for the change in current flow through resistance I52.

These partial and then more complete compensating operations together comprise a socalled follow-up or neutralizing action. It is noted the current flow through resistance I52 will not be restored precisely to its original value, but will be somewhat less than its original value because a slightly different potential on the control electrode of valve I51 is required to maintain the new output current from valve I51. The extent to which the potential drop across resistance I52 must be displaced for this purpose is very slight since the valve I51 has a narrow operating range, that is, only a. small change in potential of the control electrode is required to vary the output current throughout its entire range of variation. The valve I58 also desirably has a similar narrow operating range. The effect of this follow-up or neutralizing action is to tend to restore the potential of the control electrode of valve I58 to its original value and thereby to tend to restore the valve I5I to its original position, but since the potential drop across resistance I52 is some different value after the follow-up action, the fuel valve adjustment will be correspondingly different.

Such follow-up or neutralizing action is not permanent in effect as long as the furnace temperature is displaced from the desired value, or control point, by virtue of the operation of condenser I 55 and resistance I550. That is, so long as the furnace temperature is displaced from the desired value, such follow-up action as is produced by the valve I51 is transient or discontinuous in its eflect and will gradually diminish as the condenser I55 discharges through the resistance I55a. Such discharging of the condenser I55 operates to restore the potential of the control electrode of valve I51 to its original value, namely that maintained thereon by the battery I59, and therefore, the compensating change in output current from valve I51 through resistance I52 is gradually control electrodeof valve I58. This operates to diminished as the condenser I55 discharges. The result of this operation is that the poten-- tial of the control electrode of valve I58 is gradually changed in the same direction as the original change in potential thereof, and consequently, additional fuel valve adjustment will be made in a corresponding manner. Since the charging and discharging of condenser I55 through resistance I55a is gradual, such additional adjustments of valve II will also be gradual.

Such additional opening adjustments of the valve I5I will be continuously and gradually effected as long as the furnace temperature is displaced from the desired value and moreover, will be effected at a rate corresponding to the character of the operation being controlled. Such additional fuel valve adjustments are known to those skilled in the art as compensating or resetting adjustment and are effective to prevent stabilization of the furnace temperature at a value displaced from the desired con-.

trol point and consequently are effective to return the furnace temperature to the desired value. The rate of those adjustments may be varied, as desired, by adjustment of the value of resistance I55a by knob I 61, and accordingly, the apparatus of the present invention may be suited to the characteristics of the operation that is to be controlled.

On a decrease in the furnace temperature and a consequent reduction in the potential drop across the resistance I52 followed by a prolonged period in which there is a tendency for no further temperature change, the potential drop across resistance I52 will be reduced, and therefore, the potential applied to the control network I53 by conductors I63 and I64 will be reduced. This immediately effects a reduction in the negative potential applied to the control electrode of valve I 58 and thereby an increase in the energizing current to the solenoid I54, which increase in current, in turn, effects an opening adjustment of the valve I5I. Simultaneously the negative potential on the control electrode of valve I51 is reduced and as a result the supply of current from valve I51 to resistance I52 is increased. This increase in current to the resistance I52 tends to compensate for the change in potential drop thereacross caused by the furnace temperature change. Due, however, to a transient potential drop produced across the parallel connected condenser I56 and resistance I56a, the compensating change in output current from valve I51 through resistance I52 will be smaller than it would be if the condenser I56 and resistance I56a were omitted. After a predetermined interval the condenser I56 will be completely'discharged through the resistance I56a and thereby the potential across these elements will be reduced to zero. That potential is not simply dissipated, however, but will gradually appear across condenser I56 and the parallel connected condenser I55 and resistance I55a, dividing in accordance with the relative capacities of the condensers I55 and I56.

In this manner the potential of the control electrode of valve I51 is further reduced and as a result an additional change in the output current from the valve I51 through resistance I52 will be made. The effect of such additional neutralizing or follow-up action is to further restore the potential drop across resistance I 52 to its original value and consequently to effect an increase in the negative potential applied to the effect a decrease in the energization of solenoid I54, and hence, a closing adjustment of the valve I5I. The condenser I56 and resistance I56a are desirably so proportioned in relation to the other circuit components that the initial fuel valve opening adjustments are made during the beginning of the furnace temperature cycle of variation. The magnitude of the initial valve opening adjustment may be varied by adjusting the value of resistance I56a, for example by manipulation of knob I56b.

Thus it will be noted that on a furnace temperature change, followed by a prolonged period during which no further change in temperature takes place, an initially large opening adjustment of the fuel valve I5I will be made which is followed at the end of a suitable interval by a reduction in that valve opening adjustment, and thereafter, resetting adjustments of the valve I5I will be effected in the manner described until the furnace temperature has returned to the desired value thus overcoming any tendency for stabilization below the control point.

While the condition considered in the foregoing explanation has been that wherein the furnace temperature has dropped to a value below the control point and then tends to be stabilized at that lower value, it will be apparent that in actual operation, the variations in furnace temperature will be gradual. Under this condition the current flow through resistance I52 will tend to gradually decrease in value for the case when the temperature is falling and there will be a tendencyfor the initial magnification in the adjustment of valve I5I to build up to its maximum value. Such tendency is counteracted by the discharging of condenser I55 through resistance I 56a, and therefore, the magnification will assume some intermediate value depending upon the rate of current decrease in resistance I52. The magnification of the adjustment of the fuel valve I5I will thus vary in proportion to the rate of change of current flow through resistance I52 and will be larger when that rate of current change is larger since the potential on condenser I56 will discharge to a greater extent on slow rates of such current change than on rapid rates of change.

The effective magnification in the adjustment of the fuel valve I 5I is thus seen to be one which may be expressed mathematically in terms of rate of change since the magnitude of the magnification obtained is proportional to the rate of change in the condition being controlled.

While the control provisions of Fig. 12 have been illustrated in connection with the measuring arrangement of Fig. 1, it will be understood that these control provisions may also be adapted with equal facility with the measuring arrangements of Figs. 7--l1.

In Fig. 13 I have illustrated a modification of the control provisions of Fig. 12 wherein the fuel valve adjustments are effected by a rotating type reversible electrical motor I69 having a pair of windings I10 and HI which are adapted tube selectively energized to produce opening and close ing adjustments of the fuel valve I5I. The selective energization of the motor windings I10 and I" is controlled in accordance with the deflections of a movable contact I12 of a differential relay I13, The relay I13 includes a pair of coils I14 and I15 and also a pair of armatures I16 and I11 which extend into the relay coils I14 and I15, respectively. The movable contact I12 .windings I and Ill.

is carried by a lever I18 which is pivoted at a point I18 and carries the armatures I18 and I11. The armatures I18 and Ill are connected to the lever I18 at points on opposite sides of the pivot I19 so that when one or the other of the coils I14 and I16 is energized to a greater extent than the other, the lever I18 is rotated around its pivot point to thereby effect movement of the contact, I12 into engagement with one or the other of a pair of contacts I80 or I8I. When both of the coils are equally energized, the lever I18 is adjusted to a position wherein the contact I12 is intermediate the contacts I80 and Ill. The relay coil I14 is connected in the output circuit of the electronic valve I 58 and the other relay coil is connected through an adjustable resistance I82 to the alternating current supply conductors L and L The movable contact I12 is connected by a conductor I83 to the alternating supply conductor L and the contact I80 and I8I are connected to a respective terminal of the motor The other ends of the motor windings are connected to the supply line L. Thus, for example, on an increase in the energizing current through the relay I14 and a consequent deflection of the contact I12 with the contact I8I, the motor will be energized for rotation in the direction to effect an opening adjustment of the fuel valve 2. Thereafter, the resistance I82 will be adjusted by the motor I88 to decrease the resistance in circuit with the relay winding I15 and thereby increase the energize.- tion of winding I15.

Subject matter disclosed in this application and not claimed therein is disclosed and is being claimed in application Serial No. 301,174 filed by Harry S. Jones on October 25, 1939, for Control apparatus, issued into Patent 2,282,726 on May 12, 1942.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. Self balancing apparatus for measuring the magnitude of a unidirectional potential in a low resistance circuit including a unidirectional potential of relatively fixed magnitude, a unidirectional potential of variable magnitude, means to oppose said second and third mentioned potentials to derive a differential potential, means to oppose said differential potential to said first mentioned potential to derive a second differential potential, and physically stationary means controlled by said second differential potential to regulate the magnitude of said third mentioned potential to reduce the magnitude of said second mentioned differential potential.

2. Self balancing apparatus for measuring the magnitude of a unidirectional potential in a low resistance circuit including a unidirectional potential of relatively fixed magnitude, a unidirectional potential of variable magnitude, means to oppose said second and third mentioned potentials to derive a differential potential, means to oppose said differential potential to said first mentioned potential to derive a second differential potential, and physically stationary means controlled directly and electrically by said second difierential potential to regulate the magnitude of said third mentioned potential to reduce the magnitude of said second mentioned diii'erential potential.

3. Self balancing apparatus for measuring the magnitude of a unidirectional potential in a low resistance circuit including a unidirectional potential of relatively fixed magnitude, a unidirectional potential of variable magnitude, a circuit to oppose said second and third mentioned potentials, a resistance so connected in said last mentioned circuit that current flows through said resistance in one direction or the other to create a potential drop thereacross of one polarity or of opposite polarity depending upon which of said two first mentioned potentials is the greater, means to oppose the potential drop across said resistance to said first mentioned unidirectional potential to derive a differential potential, and physically stationary means controlled by said differential potential to regulate the magnitude of said third mentioned potential and thereby the current flows through said resistance to reduce the magnitude of said differential potential.

4. Self balancing apparatus for measuring the magnitude of a unidirectional potential in a low resistance circuit including a unidirectional potential of relatively fixed magnitude, a unidirectional potential of variable magnitude, a circuit to oppose said second and third mentioned potentials, a resistance so connected in said last mentioned circuit that current flows through said resistance in one direction or the other to create a potential drop thereacross of one polarity or of opposite polarity depending upon which of said two first mentioned potentials is the greater, means to oppose the potential drop across said resistance to said first mentioned unidirectional potential to derive a differential potential, physically stationary means controlled by said differ-i ential potential to regulate the magnitude of said third mentioned potential and thereby the current flow through said resistance to reduce the magnitude of said differential potential, and means to measure the magnitude of the current flow through said resistance.

5. Self balancing apparatus for measuring the magnitude of a unidirectional potential in a low resistance circuit including a unidirectional potential of predetermined magnitude, a resistance, a circuit to impress said second mentioned potential on said resistance to create a current flow through the latter, means to oppose the potential drop produced across said resistance by said current flow to said first mentioned potential to derive a differential potential, and physically stationary means controlled by said differential potential to divert part of the current flow through said resistance or to supplement the current flow through said resistance as required to reduce said differential potential.

6. Self balancing apparatus for measuring the magnitude of a unidirectional potential in a low resistance circuit including a unidirectional potential of predetermined magnitude, a resistance, a circuit to impress said second mentioned potential on said resistance to create a current flow through the latter, means to oppose the potential drop produced across said resistance by said current flow to said first mentioned potential to derive a differential potential physically stationary means controlled by said differential potential to divert part of the current flow through said resistance or to supplement the current flow through said resistance as required to reduce said differential potential and means to measure the current flow through said resistance.

7. Self balancing apparatus for measuring the magnitude of a variable unidirectional potential in a low resistance circuit including a source of adjustable unidirectional potential, a resistance, a circuit to impress said adjustable potential on said resistance to create a current fiow'through the latter, means to oppose the potential drop produced across said resistance, by said current flow to said first mentioned potential to derive a differential potential, physically stationary means controlled by said differential potential to divert part of the current fiow through said resistance or to supplement the current flow through said resistance as required to reduce said differential potential, and means to adjust said adjustable unidirectional potential 50 that said differential potential is zero when said first mentioned potential is within its normal range of variation.

8. Self balancing apparatus for measuring the magnitude of a variable unidirectional potential in a low resistance circuit including a source of adjustable unidirectional potential, a resistance, a circuit to impress said adjustable potential on said resistance to create a current flow through the latter, means to oppose the potential drop produced across said resistance by said current flow to said first mentioned potential to derive a differential potential, physically stationary means controlled by said differential potential to divert part of the current flow through said resistance or to supplement the current fiow through said resistance as required to reduce said differential potential, means to adjust said adjustable unidirectional potential so that said differential potential is zero when said first mentioned potential is within its normal range of variation, and means to measure the current flow through said resistance.

9. Self balancing apparatus for measurin the magnitude of a unidirectional potential including a unidirectional potential of predetermined magnitude, a resistance, a, circuit to impress said second mentioned potential on said resistance to create a current flow through the latter, means to oppose the potential drop produced across said resistance by said current flow to said first mentioned potential to derive a differential potential, means to convert said difierential potential into. a fluctuating potential, means to amplify said fluctuating potential, and an electronic reconverter controlled by said amplified potential to divert part of the current flow through said resistance or to supplement the current flow through said resistance as required to reduce said differential potential.

10. The combination of claim 9 wherein said electronic reconverter includes a pair of electronic valves each of which have an input circuit and an output circuit, separate resistances connected in the output circuits of said valves, the path including one valve and its associated resistance being connected in parallel with the path in cluding the other valve and its associated resistance, a source of current for energizing said paths, and means to apply said amplified potential to the input circuits of said valves, and wherein said second mentioned potential and said first mentioned resistance are connected in parallel with each other and this parallel connection is connected between a point on one of said parallel paths and a point on the other of said parallel paths.

11. The combination of claim 9 wherein said electronic reconverter includes a pair of electronic valves each of which have an input circuit and an output circuit, separate resistances connected in the output circuits of said valves, the path including one valve and. its associated resistance being connected in parallel with the path including the other valve and its associated resistance, a source of current for energizing said paths, and means to apply said amplified potential to the input circuits of said valves, and wherein said second mentioned potential and said first mentioned resistance are connected in parallel with each other and this parallel connection is connected between a point on one 0! said second mentioned resistances and a point on the other of said second mentioned resistances, one of said points being adjustable. l

12. Self-balancing apparatus for measuring the magnitude of a unidirectional otential including a source of adjustable unidirectional potential, a resistance, a circuit to impress said adjustable potential on said resistance to create a current flow through the latter, means to oppose the potential drop produced across said resistance by said current flow to said first mentioned potential to derive a difi'erential potential, means to adjust said adjustable potential, means to convert said differential potential into a fluctuating potential, means to amplify said fluctuating potential, an electronic reconverter to divert part of the current flow through said resistance or to supplement the current fiow through said resistance as required to reduce aid differential potential, said electronic reconverter including a pair of electronic valves each of which have an input circuit and an output circuit, means to apply said amplified potential to the input circuits of said valves to control their conductivity, separate resistances connected in the output circuits of said valves, the path including'one valve and its associated resistance being connected in parallel with the path of the other valve and its associated resistance, a source of current for energizing said paths, means connecting said second mentioned potential and said first mentioned resistance in parallel with each other between a point on one of said second mentioned resistances and a point on the other of said second mentioned resistances, the potential difference between said points being opposed to said second mentioned potential and one of said points being adjustable, and means to adjust said adjustable point and said adjustable potential in correspondence with each other to render the current flow through said first mentioned resistance from said points zero when said first mentioned potential is within its normal range of variation.

13. Self-balancing apparatus for measuring the magnitude of a. unidirectional potential in a low resistance circuit including a unidirectional potential of relatively fixed magnitude, a unidirectional potential of variable magnitude, a circuit to oppose said second and third mentioned potentials, a resistance connected in said last mentioned circuit in series with said potentials whereby current fiows through said resistance in one direction or the other to create a potential drop thereacross of one polarity or of opposite polarity depending upon which of said two first mentioned potentials is the greater, means to 

